Voltage-sensitive calcium channels mediate the entry of calcium into many types of cells, including cardiac, skeletal, and smooth muscle as well as excitable and secretory cells. Tsien, R. W. Annu. Rev. Physiol. 45:341-398, 1983; Reuter, H. Nature 301:569-574, 1983. Dihydropyridines, such as nitrendipine, nifedipine, and PN200-110, bind with high affinity and specificity to putative calcium channels and have been used extensively as tools for the purification and characterization of these components. See e.g.,Glossmann, H., Ferry, D. R., Lubbecke, F., Mewes, R. and Hofmann, F. Trends Pharmacol. Sci. 3:431-437, 1982; Reuter, H., Porzig, H., Kokubun, S. and Prod'hom, B. Trends Neurosci. 8:396-400, 1985; and Godfraind, T., Miller, R. and Wibo, M. Pharm. Reviews 38:321-416, 1986.
Transverse tubules of skeletal muscle contain high concentrations of dihydropyridine-sensitive calcium channels, yet their function at these sites remains unclear (Almers, W., Fink, R. and Palade, P. T. J. Physiol. 312:177-207, 1981; Chiarandini, D. J. and Stefani, E. J. Physiol. 335:29-40, 1983; Schwartz, L. M., McClesky, E. W. and Almers, W. Nature 314:747-751, 1985). Highly purified preparations of dihydropyridine binding complex isolated from skeletal muscle exhibit voltage sensitive (Flockerzi, V., Oeken, H-J., Hofmann, F., Pelzer, D., Cavalie, A. and Trautwein, W. Nature 323:66-68, 1986) or agonist-stimulated (Curtis, B. M. and Catterall, W. A. Biochemistry 25:3077-3083, 1986) calcium channel activity after reconstitution. There is general agreement that a polypeptide of M.sub.r 170,000, which upon reduction is converted to a protein of M.sub.r 140,000, is a major component of this complex (Curtis, B. M. and Catterall, W. A. Biochemistry 23:2113-2118, 1984; Flockerzi, V., Oeken, H-J. and Hofmann, F. Eur. J. Biochem. 161:217-224, 1986; Striessnig, J.,-Moosburger, K., Goll, A., Ferry, D. and Glossman, H. Eur. J. Biochem. 161: 603-609, 1986; Borsotto, M., Barhanin, J., Fosset, M. and Lazdunski, M. J. Biol. Chem. 260:14255-14263, 1984). Other proteins of M.sub.r 50,000 and M.sub.r 33,000 have also been shown to be present in some preparations (Flockerzi, V., Oeken, H-J., Hofmann, F., Pelzer, D., Cavalie, A. and Trautwein, W. Nature 323:66-68, 1986; Curtis, B. M. and Catterall, W. A. Biochemistry 23:2113-2118, 1984). However, there is considerable dispute as to the association of these components with the channel (Vandaele, S., Fosset, M., Galizzi, J-P. and Lazdunski, M. Biochemistry 26:5-9, 1987).
The development of the monoclonal technique has greatly facilitated the search for phenotype heterogeneity of tumors and normal tissues and the recognition of tumor associated antigens. However, the number of tumor associated antigens identified is still limited. Identification of additional tumor-associated antigens will improve differential diagnosis and classification of tumors which in turn will provide better modes of treatment of tumors.